Pressurized water nuclear power stations are well known. Briefly, they comprise a reactor containing, in a vessel, fuel assemblies formed of fuel rods containing the fissile material ; in some of them mobile control rods or bars are inserted containing a neutron absorbing material. The control bars of a fuel assembly, controlled together, form a control cluster. The fuel assemblies are plunged into the pressurized water which flows through a primary circuit having several primary loops each including a primary pump and a steam generator. One of these loops also comprises a pressurizer maintaining the water pressure in the reactor. The pressurized water serves as moderating and heat-carrying fluid. Furthermore, it contains boron in solution, a neutron absorbing material serving, like the control clusters, for controlling the operation of the reactor.
The steam generators supply steam to a secondary circuit essentially comprising a turbine driving an alternator, a condenser and pumps.
The reactivity is a measurement of the evolution of the chain reaction in the core of the reactor. In this chain reaction, the neutrons produced by the fission of heavy nuclei, slowed down by the moderator which is the pressurized water of the primary circuit, absorbed to a greater or lesser extent by the control bars and the boron in solution, in their turn cause new fissions. The factor, called k, by which the number of fissions from one generation to the next is multiplied, is generally equal to 1. It may be temporarily greater than 1. The positive difference of k with respect to 1 is called reactivity. It is reckoned in pcm parts per hundred thousand). With a non zero reactivity, the chain reaction tends to increase. At other moments, the factor k may be less than 1, the reactivity is negative and we then speak of antireactivity. In this case, the reaction tends to die out.
The power of the reactor is adjusted by adjusting the reactivity, in fact by adjusting the position of the control bars and/or the boron concentration. To increase the power, a positive reactivity is provided. The reaction increases. The temperature increases in the reactor and the density of the water of the primary circuit decreases. Its moderating effect diminishes which is equivalent to providing antireactivity, which finally counterbalances the reactivity. The reactor is stabilized then at an increased power level. To reduce the power, the reverse operation is carried out.
The reactor may thus deliver the thermal power which is required of it, generally to cope with the electricity requirements of the grid to which the power station is coupled.
In the considerations which govern the choice between the two methods of controlling a nuclear reactor, using control bars or the boron level, it should first of all be mentioned that, though action on the control bars has immediate effects, the action by boron in solution is comparatively slower.
Moreover, the increase of the boron in solution concentration requires means for storing and injecting boric acid, whereas its decrease requires dilution means and, especially, means for processing and storing the effluents, means which are all the more considerable and expensive since recourse will be had more often and for longer periods to the action using boron in solution.
Thus, there is a tendency to use boron in solution only for correcting the long term effects on the reactivity of the operation of the reactor, namely essentially the xenon effect and the ageing of the fuel.
Controlling the thermal power delivered by the reactor to correspond to the requirements of the electric grid is thus preferably carried out using the control bars. But the insertion of the control bars prejudicially affects the axial distribution of the power produced in the reactor. Temperature inequalities result in the core of the reactor with, more particularly, an increased wear of the fuel at the hottest points and a localized production of xenon, which factors have a restrictive influence on the procedure for controlling the reactor and involve a correlative recourse to the action on the level of the boron in solution.
Now, with the increase in the participation of nuclear power stations in the total production of electricity it has become necessary for the nuclear power stations, initially used as basic power stations with a substantially constant production level, to be used as a function of the load, with a production level following a daily curve and even in pilot controlled mode, by remote adjustment, the production level conforming to an arbitrary curve, thus increasing the control actions, with the unfavorable consequences mentioned above. Thus, attempts have been made to find control methods using the control bars in which the distortion of the axial power distribution is reduced and its prejudicial effects limited.
Furthermore, in order to cope with the requirements of the grid, it may be necessary for the power station to have a certain capacity to return rapidly to power, when it is operating at intermediate power. This capacity can only be ensured if, at the intermediate powers, groups of control bars are sufficiently inserted. Thus, French patent No. 2,395,572 disclosed a method for driving a nuclear reactor in which, in order to control the reactivity effects due to the power variations, depending on the power required at the turbine only, groups are displaced formed of absorbing material clusters one at least of which has reduced antireactivity, so as to vary the power of the reactor, as well as a group, called group R, formed of very absorbing clusters, as a function of the difference existing at all times between the mean temperature of the core of the reactor and a reference temperature, which is a function of the required power level, the action on the concentration of the boron in solution serving for maintaining the group R within a certain range, in addition to correcting the long term reactivity effects.
The need for a rapid return to power is complied with in the control method of patent by means of the first control bar assembly whose position is defined by the power required at the turbine. The later French Patent No. 2,493,582 disclosed a method for controlling a nuclear reactor by the combined displacement, in the core of this reactor, of groups of control bars, so that the disturbances to the axial power distribution are always limited, which avoids having recourse to boron in solution, whose concentration is then adjusted only in order to compensate for the effects of the release of xenon and the ageing of the fuel rods.
In this control method of this patent the distinction between power control groups and temperature regulation group R disappears. The position of the power control groups is continually variable following a complex program.
In the control method of this patent the need for rapid return to power will on the other hand only be satisfied to the extent that the reactivity which the control bars present in the core may cause, at the time of their withdrawal, is sufficient to allow the desired return to power. The control of this available reactivity will for example serve to avoid extraction of the control bars under the effect of the regulation of the mean temperature, when the level of poisoning of the core by xenon increases, by acting on the boric acid concentration in the core.